1. Field of the Invention
This invention relates to a process for developing electrostatic latent images which may be produced by electrophotographic methods or electrostatic recording methods and an apparatus therefor.
2. Description of the Prior Art
Heretofore, there have been known various photographic methods, recording methods and printing methods which comprise forming electric latent images (electrostatic latent images) and visualizing the latent images by using colored fine particles called "toner", in the field of image forming techniques.
For example, representative electrophotographic methods are disclosed in U.S. Pat. Nos. 2,297,691; 3,666,363; and 4,071,361. In general, electrophotographic methods are conducted by forming electric latent images on a photosensitive member composed of a photoconductive material, developing the latent image with a toner, if desired, transferring the toner images to a receiving member such as paper, and fixing the toner images by heat, pressure or a solvent vapor to produce copies. As a method for visualizing electrostatic latent images with a toner, there may be mentioned a magnetic brush developing method as disclosed in U.S. Pat. No. 2,874,063, a cascade developing method as disclosed in U.S. Pat. No. 2,618,552 and a powder cloud developing method as disclosed in U.S. Pat. No. 2,221,776.
Developers used for these developing methods may be divided into two groups, that is, one component system and two component system.
A one component system developer is composed of colored fine particles (toner) capable of being selectively attracted or repulsed by an electrostatic charge. A two component system developer is composed of the colored fine particles (toner) and a carrier material such as iron powders, glass beads and the like.
The one component system developer can develop electrostatic latent images by electric charges induced by a conductor such as a magnetic metal sleeve for carrying the developer while the two component system developer can develop electrostatic latent images by triboelectric charges.
Among various dry developing methods, as a developing method using a one component system developer, there is known a charge induction developing method, for example, as disclosed in U.S. Pat. No. 3,166,432. The charge induction developing method comprises attaching conductive toner particles to a developer dispensing member and contacting the developer dispensing member carrying the toner particles with an electrostatic latent image bearing surface to develop the latent images.
According to this developing method, when the toner particle layer on the developer dispensing member faces the electrostatic latent images, electric charges which are opposite in polarity to the electric charges of the electrostatic latent images are induced in the toner particles because the toner particles are conductive, and the resulting electric attraction between the electrostatic latent images and the oppositely charged toner particles carries out the development of the latent images.
In such charge induction developing method, since the developer is composed of toner particles only, it is not necessary to control the mixing ratio (i.e. concentration of toner) of toner particles and a carrier material and further an agitating procedure for charging the developer is not necessary so that the developing device can be simple and small in size. Neither occurs degradation of the developed image quality which is often caused by deterioration of a carrier as time passes away.
Though charge induction developing methods have the above mentioned advantages, the methods have been heretofore, practically used only for a process comprising forming electrostatic latent images on a photosensitive paper having a coating of a photosensitive material such as zinc oxide and the like, directly developing the latent images and fixing the developed images.
However, in the field of the image forming techniques thee have been recently used widely copying processes containing a step in which developed images are transferred to a receiving material (for example, ordinary paper), and it is very difficult to apply the above mentioned charge induction developing method to such copying processes and moreover there are various disadvantages. In other words, an electrostatic transferring procedure of developed images is effected with difficulty because a conductive toner is used.
For example, when a corona transfer method is conducted using a conductive toner (a corona transfer method comprises contacting toner images on an electrostatic latent image bearing member with a receiving material and charging from the rear side of the receiving material by corona discharge to transfer the toner images to the receiving member), the toner particles are charged in the same polarity as the receiving member by a slight corona electric current passing through the receiving material. As the result, the transfer is not sufficiently effected and moreover the developed images become irregular by the repulse between the toner particles because the whole toner layer is charged in the same polarity.